Allow changing focus between embedded activities in a Task

The focused window was determined by looking up the focusable windows
from top to bottom.

In order to switch the focus between the embedded activities in the
same leaf Task without reordering the position of the activities, the
focused app can also be updated based on the window that user was
touched, and the focusable windows that were on other TaskFragment
in the same Task cannot be focused.

Bug: 189385926
Test: changing focused app by touching embedded activity
Change-Id: Id84461572cc768f474ffa313da1c10ff4010408e